1. Field of the Invention
The present invention relates to a two-way optical amplifier module capable of amplifying two-way signal light, i.e., upward (in the direction of higher hierarchy node) and downward signal light, in a single optical amplifier.
2. Description of the Related Art
Optical components such as isolators and EDF (Erbium-Doped Fiber, hereinbelow abbreviated as "Er") are usually necessary on both the upward transmission path and downward. transmission path when transmitting and amplifying two-way signal light, i.e., upward signal light and downward signal light, and this requirement has prevented both reduction in size and reduction of costs.
The large number of parts further contributes to the problems of deterioration due to insertion loss in the signal light as well as deterioration of NF (Noise Figure) due to insertion loss in preceding sections.
There have been a number of proposals regarding this type of two-way optical communication. In Japanese Patent Laid-open No. 294646/92 for example, an optical multiplexer and optical demultiplexer are provided at both the preceding section and following section of an optical amplifier, a first optical isolator is provided at the amplifier for upward signal light, and a second optical isolator and optical filter are provided at the amplifier for downward signal light.
Alternatively, Japanese Patent Laid-open No. 224253/93 discloses a case in which optical multiplexers and optical demultiplexers are provided at both the preceding section and following section of an optical amplifier, optical isolators are provided on the transmission paths (by means of optical fiber) that are divided by the optical demultiplexers, and signal light is multiplexed by the optical multiplexers and used as one transmission path.
In the case of these publications, however, the use of three components, i.e., the optical isolators, optical multiplexers, and optical demultiplexers, at the preceding section of the optical amplifier results in a large loss at the input section and a deterioration in noise figure.
In this regard, a reduction of insertion loss can be achieved by reducing the number of components in the input section to either an optical circulator and optical multiplexer or a first optical circulator.
In the above-described prior art, the overall insertion loss is inevitably great because signal light passes through a total of eight components.
A reduction in the number of components, and a resulting reduction in insertion loss and reduction in size, can be achieved by replacing the optical isolators, optical multiplexers, and optical demultiplexers with optical circulators.
As for the technology for realizing upward signal light/downward signal light two-way communication using a single optical amplifier, Japanese Patent Laid-open No. 194518/91 discloses a case in which signal light is received and transmitted through rare-earth element-added optical fibers using optical circulators, reflected light being passed through the optical circulators and radiated to the outside so as not to return to the rare-earth element-added optical fibers; and Japanese Patent Laid-open No. 69579/94 discloses a case in which exciting light together with signal light is received by way of an optical circulator to the first port of an optical coupler and there split into two branches, each branch is supplied from the second and third ports of the optical coupler to first and second terminals of optical amplifying medium, the branches of signal light supplied to the first and second terminals of the optical amplifying medium are directed in mutually differing directions within the optical amplifying medium, and an optical amplifying effect is realized by means of stimulated emission.
Finally, Japanese Patent Laid-open No. 5806/97 discloses an optical amplifier that effects two-way communication wherein, in order to reduce the number of components and accompanying loss, exciting light is supplied to a rare earth-added optical fiber by way of an optical circulator, which has a forward transfer characteristic in the direction of first terminal to third terminal and in which the rare earth-added fiber is connected to the second terminal and a signal light output terminal is connected to the third terminal, and the signal light amplified by the rare-earth-added fiber and transmitted from the signal light output terminal.
All of the cases of the prior art disclosed in the above-described publications, however, still leave room for reduction of the number of components, as already described. In addition, the examples of the prior art cannot be considered satisfactory from the viewpoint of insertion loss and NF deterioration; nor can they be considered satisfactory from the standpoint of reduction of size and costs.